The first ankyrin-like repeat is the minimum indispensable key structure for functional assembly of homo- and heteromeric TRPC4/TRPC5 channels

Novel Targets for Stroke Therapy: Special Focus on TRPC Channels and TRPC6

Stroke remains a leading cause of death, disability, and medical care burden worldwide. However, transformation from laboratory findings toward effective pharmacological interventions for clinical stroke has been unsatisfactory. Novel evidence has been gained on the underlying mechanisms and therapeutic potential related to the transient receptor potential (TRP) channels in several disorders. The TRP superfamily consists of a diverse group of Ca²⁺ permeable non-selective cation channels. In particular, the members of TRP subfamilies, TRP canonical (TRPC) channels and TRPC6, have been found in different cell types in the whole body and have high levels of expression in the central nervous system (CNS). Notably, the TRPCs and TRPC6 channel have been implicated in neurite outgrowth and neuronal survival during normal development and in a range of CNS pathological conditions. Recent studies have shown that suppression of TRPC6 channel degradation prevents ischemic neuronal cell death in experimental stroke. Accumulating evidence supports the important functions of TRPC6 in brain ischemia. We have highlighted some crucial advancement that points toward an important involvement of TRPCs and TRPC6 in ischemic stroke. This review will make an overview of the TRP and TRPC channels due to their roles as targets for clinical trials and CNS disorders. Besides, the primary goal is to discuss and update the critical role of TRPC6 channels in stroke and provide a promising target for stroke prevention and therapy.

Structure-Function Relationship and Physiological Roles of Transient Receptor Potential Canonical (TRPC) 4 and 5 Channels

The study of the structure–function relationship of ion channels has been one of the most challenging goals in contemporary physiology. Revelation of the three-dimensional (3D) structure of ion channels has facilitated our understanding of many of the submolecular mechanisms inside ion channels, such as selective permeability, voltage dependency, agonist binding, and inter-subunit multimerization. Identifying the structure–function relationship of the ion channels is clinically important as well since only such knowledge can imbue potential therapeutics with practical possibilities. In a sense, recent advances in the understanding of the structure–relationship of transient receptor potential canonical (TRPC) channels look promising since human TRPC channels are calcium-permeable, non-selective cation channels expressed in many tissues such as the gastrointestinal (GI) tract, kidney, heart, vasculature, and brain. TRPC channels are known to regulate GI contractility and motility, pulmonary hypertension, right ventricular hypertrophy, podocyte injury, seizure, fear, anxiety-like behavior, and many others. In this article, we tried to elaborate recent findings of Cryo-EM (cryogenic-electron microscopy) based structural information of TRPC 4 and 5 channels and domain-specific functions of the channel, such as G-protein mediated activation mechanism, extracellular modification of the channel, homo/hetero-tetramerization, and pharmacological gating mechanisms.

TRPC-mediated Ca2+ signaling and control of cellular functions

Canonical members of the TRP superfamily of ion channels have long been recognized as key elements of Ca²⁺ handling in a plethora of cell types. The emerging role of TRPC channels in human physiopathology has generated considerable interest in their pharmacological targeting, which requires detailed understanding of their molecular function. Although consent has been reached that receptor-phospholipase C (PLC) pathways and generation of lipid mediators constitute the prominent upstream signaling process that governs channel activity, multimodal sensing features of TRPC complexes have been demonstrated repeatedly. Downstream signaling by TRPC channels is similarly complex and involves the generation of local and global cellular Ca²⁺ rises, which are well-defined in space and time to govern specific cellular functions. These TRPC-mediated Ca²⁺ signals rely in part on Ca²⁺ permeation through the channels, but are essentially complemented by secondary mechanisms such as Ca²⁺ mobilization from storage sites and Na⁺/Ca²⁺ exchange, which involve coordinated interaction with signaling partners. Consequently, the control of cell functions by TRPC molecules is critically determined by dynamic assembly and subcellular targeting of the TRPC complexes. The very recent availability of high-resolution structure information on TRPC channel complexes has paved the way towards a comprehensive understanding of signal transduction by TRPC channels. Here, we summarize current concepts of cation permeation in TRPC complexes, TRPC-mediated shaping of cellular Ca²⁺ signals and the associated control of specific cell functions.

Trpc6 inactivation confers protection in a model of severe nephrosis in rats

Abstract

Mutations in canonical transient receptor potential-6 (TRPC6) channels give rise to rare familial forms of focal and segmental glomerulosclerosis (FSGS). Here we examined a possible role for TRPC6 in the progression of chronic puromycin aminonucleoside (PAN) nephrosis in Sprague-Dawley rats, a classic model of acquired nephrotic syndromes. We used CRISPR/Cas9 technology to delete a 239-bp region within exon 2 of the Trpc6 gene (Trpc6^del allele). Trpc6^del/del rats expressed detectable Trpc6 transcripts missing exon 2, and TRPC6 proteins could be detected by immunoblot of renal cortex. However, the abundance of Trpc6 transcripts and TRPC6 protein in renal cortex was much lower than in Trpc6^wt/wt littermates, and functional TRPC6 channels could not be detected in whole-cell recordings from glomerular cells cultured from Trpc6^del/del animals, possibly because of disruption of ankyrin repeats 1 and 2. During the chronic phase of PAN nephrosis, Trpc6^del/del rats had reduced urine albumin excretion, reduced serum cholesterol and triglycerides, and improved azotemia compared to wild-type Trpc6^wt/wt littermates. Glomerulosclerosis was severe during chronic PAN nephrosis in Trpc6^wt/wt rats but was markedly reduced in Trpc6^del/del littermates. Trpc6^del/del animals also had less severe tubulointerstitial fibrosis as assessed by several biochemical and histological analyses, as well as reduced foot process effacement and glomerular basement thickening compared to Trpc6^wtt/wt controls. None of the manipulations in this study affected the abundance of TRPC5 channels in renal cortex. TRPC3 was increased in PAN nephrosis and in Trpc6^del/del rats. These data support a role for TRPC6 channels in driving an acquired form of secondary FSGS.

Key messages

We examined aminonucleoside nephrosis in rats with wild type and inactivated TRPC6.

TRPC6 channels were inactivated by CRISPR/Cas9 editing of the Trpc6 gene.

TRPC6 inactivation reduced albuminuria in the chronic but not the acute phase.

TRPC6 inactivation reduced glomerulosclerosis and ultrastructural changes.

TRPC6 inactivation also reduced interstitial changes and renal fibrosis.

Electronic supplementary material

The online version of this article (10.1007/s00109-018-1648-3) contains supplementary material, which is available to authorized users.

Electron cryo-microscopy structure of the canonical TRPC4 ion channel

Canonical transient receptor channels (TRPC) are non-selective cation channels. They are involved in receptor-operated Ca2+ signaling and have been proposed to act as store-operated channels (SOC). Their malfunction is related to cardiomyopathies and their modulation by small molecules has been shown to be effective against renal cancer cells. The molecular mechanism underlying the complex activation and regulation is poorly understood. Here, we report the electron cryo-microscopy structure of zebrafish TRPC4 in its unliganded (apo), closed state at an overall resolution of 3.6 Å. The structure reveals the molecular architecture of the cation conducting pore, including the selectivity filter and lower gate. The cytoplasmic domain contains two key hubs that have been shown to interact with modulating proteins. Structural comparisons with other TRP channels give novel insights into the general architecture and domain organization of this superfamily of channels and help to understand their function and pharmacology.

Do TRPC channels support working memory? Comparing modulations of TRPC channels and working memory through G-protein coupled receptors and neuromodulators

Working memory is a crucial ability we use in daily life. However, the cellular mechanisms supporting working memory still remain largely unclear. A key component of working memory is persistent neural firing which is believed to serve short-term (hundreds of milliseconds up to tens of seconds) maintenance of necessary information. In this review, we will focus on the role of transient receptor potential canonical (TRPC) channels as a mechanism underlying persistent firing. Many years of in vitro work have been suggesting a crucial role of TRPC channels in working memory and temporal association tasks. If TRPC channels are indeed a central mechanism for working memory, manipulations which impair or facilitate working memory should have a similar effect on TRPC channel modulation. However, modulations of working memory and TRPC channels were never systematically compared, and it remains unanswered whether TRPC channels indeed contribute to working memory in vivo or not. In this article, we review the effects of G-protein coupled receptors (GPCR) and neuromodulators, including acetylcholine, noradrenalin, serotonin and dopamine, on working memory and TRPC channels. Based on comparisons, we argue that GPCR and downstream signaling pathways that activate TRPC, generally support working memory, while those that suppress TRPC channels impair it. However, depending on the channel types, areas, and systems tested, this is not the case in all studies. Further work to clarify involvement of specific TRPC channels in working memory tasks and how they are affected by neuromodulators is still necessary in the future.

TRPC Channels in Health and Disease

This chapter offers a brief introduction of the functions of TRPC channels in non-neuronal systems. We focus on three major organs of which the research on TRPC channels have been most focused on: kidney, heart, and lung. The chapter highlights on cellular functions and signaling pathways mediated by TRPC channels. It also summarizes several inherited diseases in humans that are related to or caused by TRPC channel mutations and malfunction. A better understanding of TRPC channels functions and the importance of TRPC channels in health and disease should lead to new insights and discovery of new therapeutic approaches for intractable disease.

Proline-dependent and basophilic kinases phosphorylate human TRPC6 at serine 14 to control channel activity through increased membrane expression

Signaling via the transient receptor potential (TRP) ion channel C6 plays a pivotal role in hereditary and sporadic glomerular kidney disease. Several studies have identified gain-of-function mutations of TRPC6 and report induced expression and enhanced channel activity of TRPC6 in association with glomerular diseases. Interfering with TRPC6 activity may open novel therapeutic pathways. TRPC6 channel activity is controlled by protein expression and stability as well as intracellular trafficking. Identification of regulatory phosphorylation sites in TRPC6 and corresponding protein kinases is essential to understand the regulation of TRPC6 activity and may result in future therapeutic strategies. In this study, an unbiased phosphoproteomic screen of human TRPC6 identified several novel serine phosphorylation sites. The phosphorylation site at serine 14 of TRPC6 is embedded in a basophilic kinase motif that is highly conserved across species. We confirmed serine 14 as a target of MAPKs and proline-directed kinases like cyclin-dependent kinase 5 (Cdk5) in cell-based as well as in vitro kinase assays and quantitative phosphoproteomic analysis of TRPC6. Phosphorylation of TRPC6 at serine 14 enhances channel conductance by boosting membrane expression of TRPC6, whereas protein stability and multimerization of TRPC6 are not altered, making serine 14 phosphorylation a potential drug target to interfere with TRPC6 channel activity.-Hagmann, H., Mangold, N., Rinschen, M. M., Koenig, T., Kunzelmann, K., Schermer, B., Benzing, T., Brinkkoetter, P. T. Proline-dependent and basophilic kinases phosphorylate human TRPC6 at serine 14 to control channel activity through increased membrane expression.

TRPC Channel Structure and Properties

TRPC channels are the first identified members in the TRP family. They function as either homo- or heterotetramers regulating intracellular Ca²⁺ concentration in response to numerous physiological or pathological stimuli. TRPC channels are nonselective cation channels permeable to Ca²⁺. The properties and the functional domains of TRPC channels have been identified by electrophysiological and biochemical methods. However, due to the large size, instability, and flexibility of their complexes, the structures of the members in TRPC family remain unrevealed. More efforts should be made on structure analysis and generating good tools, including specific antibodies, agonist, and antagonist.

New light on ion channel imaging by total internal reflection fluorescence (TIRF) microscopy

Ion channels play pivotal roles in a wide variety of cellular functions; therefore, their physiological characteristics, pharmacological responses, and molecular structures have been extensively investigated. However, the mobility of an ion channel itself in the cell membrane has not been examined in as much detail. A total internal reflection fluorescence (TIRF) microscope allows fluorophores to be imaged in a restricted region within an evanescent field of less than 200 nm from the interface of the coverslip and plasma membrane in living cells. Thus the TIRF microscope is useful for selectively visualizing the plasmalemmal surface and subplasmalemmal zone. In this review, we focused on a single-molecule analysis of the dynamic movement of ion channels in the plasma membrane using TIRF microscopy. We also described two single-molecule imaging techniques under TIRF microscopy: fluorescence resonance energy transfer (FRET) for the identification of molecules that interact with ion channels, and subunit counting for the determination of subunit stoichiometry in a functional channel. TIRF imaging can also be used to analyze spatiotemporal Ca(2+) events in the subplasmalemma. Single-molecule analyses of ion channels and localized Ca(2+) signals based on TIRF imaging provide beneficial pharmacological and physiological information concerning the functions of ion channels.

Identification of a membrane-targeting domain of the transient receptor potential canonical (TRPC)4 channel unrelated to its formation of a tetrameric structure

Canonical transient receptor potential (TRPC) channels are Ca(2+)-permeable nonselective cation channels that are activated by a wide variety of stimuli, including G protein-coupled receptors (GPCRs). The TRPC4 channel is expressed in a punctate distribution in the membrane. To identify the regulating region of the channel trafficking to the membrane, we generated deletion mutants of the TRPC4 channel. We determined that when either region that was downstream of the 20 amino acids of the N terminus or the 700-730 amino acids was deleted, the mutants were retained in the endoplasmic reticulum. By coexpression of the wild-type TRPC4 with deletion mutants, we found that the 23-29 amino acids of the N terminus regulate a membrane trafficking. Additionally, by the fluorescence resonance energy transfer (FRET) method, we found that the regions downstream of the 99 amino acid region of the N terminus and upstream of the 730 amino acid region in the C terminus produce assembly of the TRPC4 tetramers. We inferred the candidate proteins that regulate or interact with the 23-29 domain of TRPC4.

The phosphatidylinositol(4,5)bisphosphate-binding sequence of transient receptor potential channel canonical 4α is critical for its contribution to cardiomyocyte hypertrophy

Cardiomyocyte hypertrophy requires a source of Ca(2+) distinct from the Ca(2+) that regulates contraction. The canonical transient receptor potential channel (TrpC) family, a family of cation channels regulated by activation of phospholipase C (PLC), has been implicated in this response. Cardiomyocyte hypertrophy downstream of Gq-coupled receptors is mediated specifically by PLCβ1b that is scaffolded onto a SH3 and ankyrin repeat protein 3 (Shank3) complex at the sarcolemma. TrpC4 exists as two splice variants (TrpC4α and TrpC4β) that differ only in an 84-residue sequence that binds to phosphatidylinositol(4,5)bisphosphate (PIP2), the substrate of PLCβ1b. In neonatal rat cardiomyocytes, TrpC4α, but not TrpC4β, coimmunoprecipitated with both PLCβ1b and Shank3. Heightened PLCβ1b expression caused TrpC4α, but not TrpC4β, translocation to the sarcolemma, where it colocalized with PLCβ1b. When overexpressed in cardiomyocytes, TrpC4α, but not TrpC4β, increased cell area (893 ± 18 to 1497 ± 29 mm(2), P < 0.01) and marker gene expression (atrial natriuretic peptide increased by 409 ± 32%, and modulatory calcineurin inhibitory protein 1 by 315 ± 28%, P < 0.01). Dominant-negative TrpC4 reduced hypertrophy initiated by PLCβ1b, or PLCβ1b-coupled receptor activation, by 72 ± 8% and 39 ± 5 %, respectively. We conclude that TrpC4α is selectively involved in mechanisms downstream of PLCβ1b culminating in cardiomyocyte hypertrophy, and that the hypertrophic response is dependent on the TrpC4α splice variant-specific sequence that binds to PIP2.

TRPC5

Human canonical transient receptor potential channel 5 (TRPC5) has been cloned from the Xq23 region on chromosome X as a suspect in nonsyndromic mental retardation. TRPC5 is a Ca(2+)-permeable cation channel predominantly expressed in the CNS, including the hippocampus, cerebellum, amygdala, sensory neurons, and retina. It also shows more restricted expression in the periphery, notably in the kidney and cardiovascular system. Homotetrameric TRPC5 channels are primarily activated by receptors coupled to Gq and phospholipase C and/or Gi proteins, but TRPC5 channels may also gate in a store-dependent manner, which requires other partner proteins such TRPC1, STIM1, and Orai1. There is an impressive array of other activators of TRPC5 channels, such as nitric oxide, lysophospholipids, sphingosine-1-phosphate, reduced thioredoxin, protons, lanthanides, and calcium, and many can cause its direct activation. Moreover, TRPC5 shows constitutive activity, and it is responsive to membrane stretch and cold. Thus, TRPC5 channels have significant potential for synergistic activation and may serve as an important focal point in Ca(2+) signalling and electrogenesis. Moreover, TRPC5 functions in partnership with about 60 proteins, including TRPC1, TRPC4, calmodulin, IP3 receptors, NHERF, NCS-1, junctate, stathmin 2, Ca(2+)-binding protein 1, caveolin, and SESTD1, while its desensitisation is mediated by both protein kinases A and C. TRPC5 has a distinct voltage dependence shared only with its closest relative, TRPC4. Its unique N-shaped activation curve underlined by intracellular Mg(2+) block seems to be perfectly "shaped" to trigger action potential discharge, but not to grossly interfere with the action potential shape. The range of biological functions of TRPC5 channels is also impressive, from neurotransmission to control of axon guidance and vascular smooth muscle cell migration and contractility. Recent studies of Trpc5 gene knockouts begin to uncover its roles in fear, anxiety, seizures, and cold sensing.

Physiological functions and regulation of TRPC channels

The TRP-canonical (TRPC) subfamily, which consists of seven members (TRPC1-TRPC7), are Ca(2+)-permeable cation channels that are activated in response to receptor-mediated PIP2 hydrolysis via store-dependent and store-independent mechanisms. These channels are involved in a variety of physiological functions in different cell types and tissues. Of these, TRPC6 has been linked to a channelopathy resulting in human disease. Two key players of the store-dependent regulatory pathway, STIM1 and Orai1, interact with some TRPC channels to gate and regulate channel activity. The Ca(2+) influx mediated by TRPC channels generates distinct intracellular Ca(2+) signals that regulate downstream signaling events and consequent cell functions. This requires localization of TRPC channels in specific plasma membrane microdomains and precise regulation of channel function which is coordinated by various scaffolding, trafficking, and regulatory proteins.

TRPC4- and TRPC4-containing channels

TRPC4 proteins comprise six transmembrane domains, a putative pore-forming region, and an intracellularly located amino- and carboxy-terminus. Among eleven splice variants identified so far, TRPC4α and TRPC4β are the most abundantly expressed and functionally characterized. TRPC4 is expressed in various organs and cell types including the soma and dendrites of numerous types of neurons; the cardiovascular system including endothelial, smooth muscle, and cardiac cells; myometrial and skeletal muscle cells; kidney; and immune cells such as mast cells. Both recombinant and native TRPC4-containing channels differ tremendously in their permeability and other biophysical properties, pharmacological modulation, and mode of activation depending on the cellular environment. They vary from inwardly rectifying store-operated channels with a high Ca(2+) selectivity to non-store-operated channels predominantly carrying Na(+) and activated by Gαq- and/or Gαi-coupled receptors with a complex U-shaped current-voltage relationship. Thus, individual TRPC4-containing channels contribute to agonist-induced Ca(2+) entry directly or indirectly via depolarization and activation of voltage-gated Ca(2+) channels. The differences in channel properties may arise from variations in the composition of the channel complexes, in the specific regulatory pathways in the corresponding cell system, and/or in the expression pattern of interaction partners which comprise other TRPC proteins to form heteromultimeric channels. Additional interaction partners of TRPC4 that can mediate the activity of TRPC4-containing channels include (1) scaffolding proteins (e.g., NHERF) that may mediate interactions with signaling molecules in or in close vicinity to the plasma membrane such as Gα proteins or phospholipase C and with the cytoskeleton, (2) proteins in specific membrane microdomains (e.g., caveolin-1), or (3) proteins on cellular organelles (e.g., Stim1). The diversity of TRPC4-containing channels hampers the development of specific agonists or antagonists, but recently, ML204 was identified as a blocker of both recombinant and endogenous TRPC4-containing channels with an IC50 in the lower micromolar range that lacks activity on most voltage-gated channels and other TRPs except TRPC5 and TRPC3. Lanthanides are specific activators of heterologously expressed TRPC4- and TRPC5-containing channels but can block individual native TRPC4-containing channels. The biological relevance of TRPC4-containing channels was demonstrated by knockdown of TRPC4 expression in numerous native systems including gene expression, cell differentiation and proliferation, formation of myotubes, and axonal regeneration. Studies of TRPC4 single and TRPC compound knockout mice uncovered their role for the regulation of vascular tone, endothelial permeability, gastrointestinal contractility and motility, neurotransmitter release, and social exploratory behavior as well as for excitotoxicity and epileptogenesis. Recently, a single-nucleotide polymorphism (SNP) in the Trpc4 gene was associated with a reduced risk for experience of myocardial infarction.

Novel pyrazole compounds for pharmacological discrimination between receptor-operated and store-operated Ca(2+) entry pathways

BACKGROUND AND PURPOSE

Pyrazole derivatives have recently been suggested as selective blockers of transient receptor potential cation (TRPC) channels but their ability to distinguish between the TRPC and Orai pore complexes is ill-defined. This study was designed to characterize a series of pyrazole derivatives in terms of TRPC/Orai selectivity and to delineate consequences of selective suppression of these pathways for mast cell activation.

EXPERIMENTAL APPROACH

Pyrazoles were generated by microwave-assisted synthesis and tested for effects on Ca(2+) entry by Fura-2 imaging and membrane currents by patch-clamp recording. Experiments were performed in HEK293 cells overexpressing TRPC3 and in RBL-2H3 mast cells, which express classical store-operated Ca(2+) entry mediated by Orai channels. The consequences of inhibitory effects on Ca(2+) signalling in RBL-2H3 cells were investigated at the level of both degranulation and nuclear factor of activated T-cells activation.

KEY RESULTS

Pyr3, a previously suggested selective inhibitor of TRPC3, inhibited Orai1- and TRPC3-mediated Ca(2+) entry and currents as well as mast cell activation with similar potency. By contrast, Pyr6 exhibited a 37-fold higher potency to inhibit Orai1-mediated Ca(2+) entry as compared with TRPC3-mediated Ca(2+) entry and potently suppressed mast cell activation. The novel pyrazole Pyr10 displayed substantial selectivity for TRPC3-mediated responses (18-fold) and the selective block of TRPC3 channels by Pyr10 barely affected mast cell activation.

CONCLUSIONS AND IMPLICATIONS

The pyrazole derivatives Pyr6 and Pyr10 are able to distinguish between TRPC and Orai-mediated Ca(2+) entry and may serve as useful tools for the analysis of cellular functions of the underlying Ca(2+) channels.

Canonical transient receptor potential (TRPC) 1 acts as a negative regulator for vanilloid TRPV6-mediated Ca2+ influx

TRP proteins mostly assemble to homomeric channels but can also heteromerize, preferentially within their subfamilies. The TRPC1 protein is the most versatile member and forms various TRPC channel combinations but also unique channels with the distantly related TRPP2 and TRPV4. We show here a novel cross-family interaction between TRPC1 and TRPV6, a Ca²⁺ selective member of the vanilloid TRP subfamily. TRPV6 exhibited substantial co-localization and in vivo interaction with TRPC1 in HEK293 cells, however, no interaction was observed with TRPC3, TRPC4, or TRPC5. Ca²⁺ and Na⁺ currents of TRPV6-overexpressing HEK293 cells are significantly reduced by co-expression of TRPC1, correlating with a dramatically suppressed plasma membrane targeting of TRPV6. In line with their intracellular retention, remaining currents of TRPC1 and TRPV6 co-expression resemble in current-voltage relationship that of TRPV6. Studying the N-terminal ankyrin like repeat domain, structurally similar in the two proteins, we have found that these cytosolic segments were sufficient to mediate a direct heteromeric interaction. Moreover, the inhibitory role of TRPC1 on TRPV6 influx was also maintained by expression of only its N-terminal ankyrin-like repeat domain. Our experiments provide evidence for a functional interaction of TRPC1 with TRPV6 that negatively regulates Ca²⁺ influx in HEK293 cells.

Update on vascular endothelial Ca2+ signalling: A tale of ion channels, pumps and transporters

A monolayer of endothelial cells (ECs) lines the lumen of blood vessels and forms a multifunctional transducing organ that mediates a plethora of cardiovascular processes. The activation of ECs from as state of quiescence is, therefore, regarded among the early events leading to the onset and progression of potentially lethal diseases, such as hypertension, myocardial infarction, brain stroke, and tumor. Intracellular Ca²⁺ signals have long been know to play a central role in the complex network of signaling pathways regulating the endothelial functions. Notably, recent work has outlined how any change in the pattern of expression of endothelial channels, transporters and pumps involved in the modulation of intracellular Ca²⁺ levels may dramatically affect whole body homeostasis. Vascular ECs may react to both mechanical and chemical stimuli by generating a variety of intracellular Ca²⁺ signals, ranging from brief, localized Ca²⁺ pulses to prolonged Ca²⁺ oscillations engulfing the whole cytoplasm. The well-defined spatiotemporal profile of the subcellular Ca²⁺ signals elicited in ECs by specific extracellular inputs depends on the interaction between Ca²⁺ releasing channels, which are located both on the plasma membrane and in a number of intracellular organelles, and Ca²⁺ removing systems. The present article aims to summarize both the past and recent literature in the field to provide a clear-cut picture of our current knowledge on the molecular nature and the role played by the components of the Ca²⁺ machinery in vascular ECs under both physiological and pathological conditions.

The TR (i)P to Ca²⁺ signaling just got STIMy: an update on STIM1 activated TRPC channels

Calcium is a ubiquitous signaling molecule, indispensable for cellular metabolism of organisms from unicellular life forms to higher eukaryotes. The biological function of most eukaryotic cells is uniquely regulated by changes in cytosolic calcium, which is largely achieved by the universal phenomenon of store-operated calcium entry (SOCE). The canonical TRPs and Orai channels have been described as the molecular components of the store-operated calcium channels (SOCC). Importantly, the ER calcium-sensor STIM1 has been shown to initiate SOCE via gating of SOCC. Since the discovery of STIM1, as the critical regulator of SOCE, there has been a flurry of observations suggesting its obligatory role in regulating TRPC and Orai channel function. Considerable effort has been made to identify the molecular details as how STIM1 activates SOCC. In this context, findings as of yet has substantially enriched our understanding on, the modus operandi of SOCE, the distinct cellular locales that organize STIM1-SOCC complexes, and the physiological outcomes entailing STIM1-activated SOCE. In this review we discuss TRPC channels and provide an update on their functional regulation by STIM1.

TRP Channels as Sensors and Signal Integrators of Redox Status Changes

Proteins are capable of sensing the redox status of cells. Cysteine residues, which react with oxidants, reductants, and electrophiles, have been increasingly recognized as the mediators of this redox sensitivity. Cation channels encoded by the transient receptor potential (trp) gene superfamily are characterized by a wide variety of activation triggers that act from outside and inside the cell. Recent studies have revealed that a class of TRP channels is sensitive to changes in redox status and is notably susceptible to modifications of cysteine residues, such as oxidation, electrophilic reaction, and S-nitrosylation of sulfhydryls. In this review, we focus on TRP channels, which directly sense redox status, and discuss the biological significance of cysteine modifications and the consequences of this chemical reaction for physiological responses.

TRPC6 channels and their binding partners in podocytes: role in glomerular filtration and pathophysiology

Loss or dysfunction of podocytes is a major cause of glomerular kidney disease. Several genetic forms of glomerular disease are caused by mutations in genes that encode structural elements of the slit diaphragm or the underlying cytoskeleton of podocyte foot processes. The recent discovery that gain-of-function mutations in Ca(2+)-permeable canonical transient receptor potential-6 channels (TRPC6) underlie a subset of familial forms of focal segmental glomerulosclerosis (FSGS) has focused attention on the basic cellular physiology of podocytes. Several recent studies have examined the role of Ca(2+) dynamics in normal podocyte function and their possible contributions to glomerular disease. This review summarizes the properties of TRPC6 and related channels, focusing on their permeation and gating properties, the nature of mutations associated with familial FSGS, and the role of TRPC channels in podocyte cell biology as well as in glomerular pathophysiology. TRPC6 interacts with several proteins in podocytes, including essential slit diaphragm proteins and mechanosensitive large-conductance Ca(2+)-activated K(+) channels. The signaling dynamics controlling ion channel function and localization in podocytes appear to be quite complex.

TRPC channels are necessary mediators of pathologic cardiac hypertrophy

Pathologic hypertrophy of the heart is regulated through membrane-bound receptors and intracellular signaling pathways that function, in part, by altering Ca(2+) handling and Ca(2+)-dependent signaling effectors. Transient receptor potential canonical (TRPC) channels are important mediators of Ca(2+)-dependent signal transduction that can sense stretch or activation of membrane-bound receptors. Here we generated cardiac-specific transgenic mice that express dominant-negative (dn) TRPC3, dnTRPC6, or dnTRPC4 toward blocking the activity of the TRPC3/6/7 or TRPC1/4/5 subfamily of channels in the heart. Remarkably, all three dn transgenic strategies attenuated the cardiac hypertrophic response following either neuroendocrine agonist infusion or pressure-overload stimulation. dnTRPC transgenic mice also were partially protected from loss of cardiac functional performance following long-term pressure-overload stimulation. Importantly, adult myocytes isolated from hypertrophic WT hearts showed a unique Ca(2+) influx activity under store-depleted conditions that was not observed in myocytes from hypertrophied dnTRPC3, dnTRPC6, or dnTRPC4 hearts. Moreover, dnTRPC4 inhibited the activity of the TRPC3/6/7 subfamily in the heart, suggesting that these two subfamilies function in coordinated complexes. Mechanistically, inhibition of TRPC channels in transgenic mice or in cultured neonatal myocytes significantly reduced activity in the calcineurin-nuclear factor of activated T cells (NFAT), a known Ca(2+)-dependent hypertrophy-inducing pathway. Thus, TRPC channels are necessary mediators of pathologic cardiac hypertrophy, in part through a calcineurin-NFAT signaling pathway.

Cell-cell contact formation governs Ca2+ signaling by TRPC4 in the vascular endothelium: evidence for a regulatory TRPC4-beta-catenin interaction

TRPC4 is well recognized as a prominent cation channel in the vascular endothelium, but its contribution to agonist-induced endothelial Ca²⁺ entry is still a matter of controversy. Here we report that the cellular targeting and Ca²⁺ signaling function of TRPC4 is determined by the state of cell-cell adhesions during endothelial phenotype transitions. TRPC4 surface expression in human microvascular endothelial cells (HMEC-1) increased with the formation of cell-cell contacts. Epidermal growth factor recruited TRPC4 into the plasma membrane of proliferating cells but initiated retrieval of TRPC4 from the plasma membrane in quiescent, barrier-forming cells. Epidermal growth factor-induced Ca²⁺ entry was strongly promoted by the formation of cell-cell contacts, and both siRNA and dominant negative knockdown experiments revealed that TRPC4 mediates stimulated Ca²⁺ entry exclusively in proliferating clusters that form immature cell-cell contacts. TRPC4 co-precipitated with the junctional proteins β-catenin and VE-cadherin. Analysis of cellular localization of fluorescent fusion proteins provided further evidence for recruitment of TRPC4 into junctional complexes. Analysis of TRPC4 function in the HEK293 expression system identified β-catenin as a signaling molecule that enables cell-cell contact-dependent promotion of TRPC4 function. Our results place TRPC4 as a Ca²⁺ entry channel that is regulated by cell-cell contact formation and interaction with β-catenin. TRPC4 is suggested to serve stimulated Ca²⁺ entry in a specific endothelial state during the transition from a proliferating to a quiescent phenotype. Thus, TRPC4 may adopt divergent, as yet unappreciated functions in endothelial Ca²⁺ homeostasis and emerges as a potential key player in endothelial phenotype switching and tuning of cellular growth factor signaling.

Transient receptor potential canonical 5 channels activate Ca2+/calmodulin kinase Igamma to promote axon formation in hippocampal neurons

Functionality of neurons is dependent on their compartmentalized polarization of dendrites and an axon. The rapid and selective outgrowth of one neurite, relative to the others, to form the axon is critical in initiating neuronal polarity. Axonogenesis is regulated in part by an optimal intracellular calcium concentration. Our investigation of Ca(2+)-signaling pathways involved in axon formation using cultured hippocampal neurons demonstrates a role for Ca(2+)/calmodulin kinase kinase (CaMKK) and its downstream target Ca(2+)/calmodulin kinase I (CaMKI). Expression of constitutively active CaMKI induced formation of multiple axons, whereas blocking CaMKK or CaMKI activity with pharmacological, dominant-negative, or short hairpin RNA (shRNA) methods significantly inhibited axon formation. CaMKK signals via the gamma-isoform of CaMKI as shRNA to CaMKIgamma, but not the other CaMKI isoforms, inhibited axon formation. Furthermore, overexpression of wild-type CaMKIgamma, but not a mutant incapable of membrane association, accelerated the rate of axon formation. Pharmacological or small interfering RNA inhibition of transient receptor potential canonical 5 (TRPC5) channels, which are present in developing axonal growth cones, suppressed CaMKK-mediated activation of CaMKIgamma as well as axon formation. We demonstrate using biochemical fractionation and immunocytochemistry that CaMKIgamma and TRPC5 colocalize to lipid rafts. These results are consistent with a model in which highly localized calcium influx through the TRPC5 channels activates CaMKK and CaMKIgamma, which subsequently promote axon formation.